The invention relates generally to the field of optical guided wave signal processors and, more particularly, to fiber optic devices for filtering signals.
The need for more and more data processing speed grows with increasingly complex applications. For example, in computer systems that process images from TV cameras for robotic vision, a vast amount of information is generated which must be processed. This vast quantity of information can overwhelm an ordinary computer, leading to unacceptable processing times.
One way of speeding up such signal processing is through pipeline architecture for computers. Pipelining is a hardware configuration, i.e., architecture for a computer, involving a plurality of processors linked into an array to achieve higher performance. This is done by breaking up complex, time-consuming functions into a series of simpler, shorter operations, each of which can be executed in assembly line fashion with simultaneous computation on different sets of data by the different processors. The improvement in performance is directly proportional to the number of pipeline processors in the array that are simultaneously kept busy doing part of the task. Pipelining is most effective in special purpose applications which can use array processors.
A further increase in the speed of such processing may be achieved by use of fiber optic systems in the pipeline architectures, as well as in other signal processing portions of a given system. Optical devices may be utilized to achieve such increases in data processing speed because of their excellent propagation and delay properties. The low loss and low dispersion of single-mode fiber allow signals to propagate large distances without significant attenuation or distortion.
The propagation and delay properties of single-mode fiber are particularly attractive because digital signal processing and conventional analog signal processing techniques (such as those using surface acoustic wave devices) are limited in their usefulness in applications involving signal bandwidths exceeding one or 2 gigahertz, although they are very effective at lower frequencies. Those digital and conventional analog signal processing techniques simply cannot be used in applications requiring very high data rates where it is often the case that bandwidths of 10 GHz and beyond may be needed.
The real-time processing of broad band radar signals is an example of a current application which would benefit from optical fiber devices capable of handling high frequency bandwidths. Other potential applications include the direct interfacing of fiber processors with high-speed optical communications systems. Likewise, fiber systems can perform real-time processing operations on the optical output of one or more fiber sensors, including the time-division multiplexing of slow sensor outputs to match the high data rates that can be handled by fiber systems. In either case, the ability to process broadband optical data before transduction into electrical signals can be of great practical value, and finds application in a wide variety of circumstances.
The concept of systolic arrays for computing data as signals pass through system has been proposed in the prior art for network processing. Various optical array processors which have been proposed for such purposes are described in the prior art. Among them are processors described by Caulfield et al., "Optical Implementation of Systolic Array Processors", Optics Communications, Vol. 40, No. 2, pages 86-90, Dec. 15, 1981; Casasent, "Acoustooptic Transducers in Iterative Optical Vector-Matrix Processors", Applied Optics, Vol. 21, No. 10, May 15, 1982; Taylor, "Fiber and Integrated Optical Devices for Signal Processing", SPIE Vol. 176, Guided Wave Optical Systems and Devices, II (1979), pages 17-27.
A common problem in these systems theory and signal processing areas is that of signal filtering. Depending upon the particular application, filtering may be used for purposes such as restoring a distorted signal, or extracting one signal from a combination of signals and/or noise. Viewed most simply, a filter functions to remove undesired components from an input signal, so that only the desired signal components are allowed to pass through the filter. In most signal processing areas, it is very desirable to have filters having the capability of providing a broad range of complex frequency responses, so that the filter may be utilized in any of numerous applications.
There are several different optical devices which have been utilized for filtering signals. For example, one device comprises a transversal filter, such as a tapped-delay line, whose tap weights are adjusted to realize the desired filtering option. This transversal filter is sometimes called a non-recursive system or a finite-duration impulse response (FIR) filter. In FIR filters, impulse signals applied to their inputs eventually pass entirely through the filter. These non-recursive filters introduce only zeros into the system transfer function, and are therefore also known as "all-zero filters." The principal disadvantage of FIR filters, whether of tapped-delay line or lattice type, is that obtaining sharp filtering requires many stages of individual taps or of directional couplers. This need for many stages makes sharp FIR lattice filters difficult to fabricate and relatively expensive. In addition, an analysis of this particular type of all-zero filter using the Z-transform technique establishes that using positive valued input signals it is impossible to obtain zeros in the transfer function of this filter in a wedge-shaped region of the Z-transform plane which is symmetric about the positive Z-axis, having its vertex at that plane's origin. In addition, this analysis also demonstrates that all the zeros of the transfer function may be restricted to the right-hand side of the Z-transform plane only if negative valued signals are allowed to be filtered through the system.
One type of filter of the FIR type is described in U.S. Pat. No. 4,159,418 to Marom. This patent discloses a matched filter coding device comprising a fiber optic structure involving two fiber optic waveguides carrying light in the same direction and cross coupled by a plurality of directional couplers. The fiber optic waveguides are multimode. This structure was said to have matched filter properties. The Marom structure differs from the structure of the present invention for reasons such as the direction of light propagation in the output fiber which results in no feedback recirculation between couplers in the Marom device. Marom also uses multimode fiber.
In addition to the FIR filters, there is another class of filters whose impulse response has infinite duration in time. These filters are called infinite-duration impulse response (IIR) or recursive filters. Recursive filters introduce, in general, both poles and zeros (pole-zero filter) in the transfer function. In a special case, recursive filters can introduce only poles into the transfer function, in which case the name "all-pole filter" is used. In the IIR filter, one input signal propagates along a feed-forward line to a successive coupler while another signal propagates along the feed-backward line to the prior coupler on the feed-forward line. Thus, a portion of the signal transmitted from the output of a directional coupler on the feed-forward line will return from the immediately successive coupler via the feed-backward line after an overall loop delay time interval T.
Like the FIR filter, a particular filtering function may be obtained from an IIR fiber optic lattice filter by selecting the amount of light signal coupled between the fibers and by selecting the loop transit times T associated with each of the filters' stages.
IIR fiber optic lattic filters exhibit the advantages of that general class of non-fiber optic filters, such as being capable of extremely sharp frequency response with a small number of stages. The disadvantages of this filter are the complexity of its design, concern over filter stability, and the filter's intrinsically non-linear phase response. Furthermore, analysis of the transfer function for an IIR fiber optic lattice filter establishes that the filter may include both poles and zeros whose respective locations on the Z-transform plane are interdependent. That is, if particular inputs and outputs are used on this type of filter, the filter's poles and zeros cannot be adjusted independently of each other. Alternatively, when the inputs and outputs of the filter are chosen so as to define an all-pole transfer function, with zeros located either at the origin or at infinity, then the poles of the filter may be adjusted independently of its zeros because of the location of its zeros. Such analysis also demonstrates that because such a fiber optic lattice filter is a positive system, in that light intensities are used as signals, its poles are restricted to the particular regions of the Z-transform plane determined by the number of stages in the filter.
Still another filter arrangement is the lattice filter, which is merely a fiber filter structure and which is implemented by cascading together a multiplicity of the fiber filter structures such as those which are individually described herein. Lattice structures are suitable forms for performing signal processing operations, and they have some advantageous characteristics as compared to other filter forms, such as modularity, regularity, ease of implementation and good sensitivity. Lattice structures are particularly desirable since, by varying the coupling coefficients of the lattice couplers, one can adjust the system transfer function and, therefore, shape the frequency response of the filter.
Fiber optic filters such as those described above suffer from a limitation which is not shared by other, corresponding classes of non-optic filters. Specifically, fiber optic lattice filters comprise positive systems since they employ light intensity as the signal carrying medium. With this signal medium, negative weighting of taps cannot be accomplished. Further, because the filters employ directional couplers which can only additively, rather than subtractively, transfer light signals between fibers, only positive valued signals may be applied to and processed within the filter. The absence of negativity in these fiber optic lattice filters limits the regions of the Z-transform plane in which poles and zeros of the respective filter classes may be located.
In light of the above, it becomes apparent that what is needed in the art is a fiber optic filter arrangement wherein the location of the filter's poles and zeros in the Z-transform plane may be established independently, thus increasing the range and complexity of response of a given filter. It would be a further improvement to provide such a filter arrangement which provides an overall filtering function that is unrestricted by an absence of negativity.